The present invention relates to a honeycomb filter system for trapping particulate matter in exhaust gas.
A requirement for removing particulate matter contained in exhaust gas discharged from internal combustion engines such as automobile engines, construction machinery engines, and industrial machinery stationary engines and other burning appliances from the exhaust gas has grown in light of influence on environment. In particular, regulations on removal of particulate matter discharged from diesel engines tend to be strengthened on a global basis. Because of such circumstances, a DPF (diesel particulate filter) for trapping and removing particulate matter is spotlighted.
A DPF is generally constituted of a single filter. A mode of a DPF is a honeycomb filter having porous partition walls partitioning and forming a plurality of cells functioning as fluid passages, where predetermined cells having open end portions on one side and plugged end portions on the other side and the remaining cells having plugged end portions on one side and open end portions on the other side are alternately disposed, and a catalyst is supported on the inner surfaces of the pores of the partition walls. When such a honeycomb filter is used as a DPF, fluid (exhaust gas) allowed to flow in from an end portion on one side where predetermined cells are open permeates the partition walls, flows to the remaining cell side as a permeation fluid, and further flows out from the other end portion where the remaining cells are open, and thereby particulate matter in the exhaust gas is oxidized and decomposed by a catalyst and trapped and removed by the pores. Since a filter such as the above honeycomb filter having a structure where exhaust gas permeates porous partition walls (wall-flow type filter) can have a large filtration area, such a filter can have a low filtration flow rate (partition wall permeation flow rate), low pressure loss, and good trapping efficiency of particulate matter.
A prior document having the same problem described below as the present invention is, for example, JP-A-2006-7148.
However, in a DPF where only one honeycomb filter as described above is employed, when pores are small, it is difficult to support a large amount of catalyst on the inner surfaces thereof, it is difficult for particulate matter to enter deep layers of the pores, and contact ability of the catalyst with the particulate matter is low. Therefore, there arises a problem of ineffectively oxidizing and decomposing particulate matter (sometimes referred to as PM) discharged from a diesel engine. In contrast, when a structure where pores are made large and a catalyst is supported on the inner surfaces of the pores in order to enhance contact ability of the catalyst with PM, an amount of PM passing through the enlarged pores is increased though oxidation reactivity of PM is improved, and thereby there arises a problem of deteriorating PM purification ability from a comprehensive viewpoint.